A typical elevator system comprises an elevator car and a counterweight, each suspended on opposite ends of hoist ropes disposed in an elevator hoistway. Compensating ropes are hung from the underside of the elevator car to the underside of the counterweight to balance the weight of the hoist ropes as the car and counterweight move alternatingly up and down within the hoistway. A compensating rope sheave, disposed on the bottom of the hoistway, allows the compensating ropes to pass therethrough.
A problem with compensating ropes arises in tall buildings, which tend to sway, as a result of winds acting upon the buildings. Under certain combinations of rope length and tension, the compensating ropes tend to vibrate with the building. The compensating ropes' motion may continuously gain amplitude as the result of the building sway. The problem of horizontal rope vibration tends to be worse when the elevator car is parked near top floors because the compensating ropes are the longest and the building sway, which excites the rope vibration, is greatest.
Such horizontal vibration of the compensating ropes is undesirable for a number of reasons. First, compensating ropes may get tangled with one another since elevators have many compensating ropes or may interfere with other cables in the hoistway. Second, horizontal movement of ropes limits the ability of the elevator car to travel at higher speeds, because the shortening of the vibrating ropes resulting from an elevator car traveling downward will increase the oscillations of the ropes, thereby inhibiting the ropes' ability to stay within the grooves of the compensating sheave. Third, the noise from the compensating ropes hitting the hoistway walls may frighten passengers and building occupants.
One common method for minimizing horizontal movement of compensating ropes is to increase the weight of a frame supporting the compensating sheave. The major drawback of increasing the dead weight on the compensating sheave is that the suspended dead weight becomes live load which must be supported by the elevator machine, thereby requiring increased capacity of the machine itself and the increased size of the associated powertrain hardware.
Another approach to dampen oscillations of the compensating ropes is to use a follower carriage attached to the ropes. However, this approach has the same major short-coming as the use of suspended dead weights. The elevator machine and drive must support the additional weight of the follower carriage.